Creping doctor blades are commonly used in the production of tissue. The blades have the function of detaching a paper web from a rigid, hot dryer cylinder (often called a Yankee dryer) and at the same time exert a compressive action on the web thereby creating the typical crepe structure of a tissue product.
Nowadays, the creping blade must fulfill many requirements:                The blade must overcome the adhesive forces which stick the paper web on the dryer surface, the adhesion being promoted (for purposes of drying the web) by a chemical coating applied to the dryer by means of a spray-boom.        The blade should create the desired crepe structure in the web and thereby provide the right bulk, softness and mechanical strength to the tissue. For this aspect, the geometry of the blade tip is important. A square edge blade (90 degrees) in a given creping situation will create a different tissue than a blade with a sharp edge of say 75 degrees in the same creping situation. The former situation gives a higher bulk and coarser crepe structure than do the latter.        The blade should keep the tissue parameters as constant as possible for the longest possible period of time. In other words, the wear of the blade tip and its interaction with the layer of coating chemicals on the web are important factors.        The blade should be as friendly as possible against the dryer surface. This means that any wear should predominantly or exclusively occur on the blade, rather than on the dryer surface. The surface of the dryer can be either cast iron (same material as the bulk of the cylinder, i.e. a cylinder without any surface deposit) or a metallization obtained by, for example, thermal spraying. As an example, WO 97/22729 describes a method for coating Yankee dryers.        
Creping blades are subjected to wear for different reasons. First there is sliding wear against the dryer, and second there is impact wear due to the web hitting the blade during creping. It has been found that the progressive wear of the creping blade is directly related to unwanted evolution of the tissue properties, such as changes in bulk and softness. Practical experience, after having reviewed many tissue mills, has shown that the best properties of the tissue are obtained only with a new blade. For steel blades, this period of good properties could be as short as one reel only.
In order to accommodate for such behavior (i.e. blade wear), tissue makers are specifying ranges of properties which are said to be acceptable. Nevertheless, there would be a high industrial demand for the tissue quality reached in the very first part of the first reel after a blade change. When the targeted range of tissue properties is not reachable anymore, the creping blade is changed for a new one, obtaining again the desirable characteristics but which are rapidly decreasing. Generally, steel blades of grade type such as AISI 1074 in quenched and tempered states are used. Such blades generally show rapid wear and consequently rapid changes in tissue quality, as well as possible micro-welding issues with the dryer surface and a so-called hot waving behavior.
For the reasons mentioned above, there has been made several attempts to improve the behavior of such blades by adding hard, wear resistant materials at the blade tip.
U.S. Pat. No. 3,688,336 explain the possibility to add a wear resistant material at the blade tip by a suitable method of the thermal spraying type. The desire to avoid chipping of the wear resistant material was recognized. The referenced U.S. patent proposes the solution to use a groove at the blade tip and a break-in space between the wear resistant material in the groove and the leading edge of the blade.
GB 2,128,551 discloses a multipurpose scraper which may be used as a creping blade, having an edge coated by thermal spraying in many passes with a wear resistant material from the ceramic or metal carbide families. More specifically, alumina-titania is presented. Focus is further made on flexibility and again is the need for minimum brittleness emphasized.
Other documents, such as U.S. Pat. No. 6,207,021 and U.S. Pat. No. 6,074,526, teach the possibility to create a recess on the blade tip in order to obtain an essentially constant contact surface against the dryer, and by this feature a constant scraping efficiency. Apart from the fact that such solutions are very much increasing the manufacturing costs for the blade, by virtue of elaborate and accurate grinding, such solutions are in practice exposed to blade tip failure due to hot friction wear and possible plastic flow of the reduced portion of the blade remaining at disposal for sliding wear.
Today, thermally sprayed ceramic tipped blades are used in the tissue industry. Ceramic compositions including alumina, alumina-titania and alumina-zirconia are well known in the field. The 60%/40% alumina-zirconia fulfills the basic requirements of good sliding wear against cast iron, very high fracture toughness and at the same time a relatively low hardness. Although creping blades having these features may bring benefits in terms of lifetime, they still suffer from a number of drawbacks:                1. Firstly, there is a large variation in blade lifetime due to chipping problems of the ceramic edge.        
The blade has to be removed and replaced after a lifetime which can be anywhere between 5 minutes to 12 hours. It has been observed in practice that most of the failures observed when using ceramic tipped blades occur during the very first period after a blade change. If relatively small, such chips are responsible for what is often called “tramlines” on the mother reel when winding. With increasing size of such chips in the blade, or decreasing grade of the tissue to lower grammages, the chips may cause web breaks and holes on the tissue. This impairs productivity and quality. In conjunction to this point, the clear trend to use more and more recycled fibers in tissue production leads to more and more high ash content and foreign particles being entrapped in the tissue-making process, thereby promoting even more chipping of the leading edge of the state of the art ceramic tipped creping blades.                2. Secondly, another limitation for such state of the art, thermally sprayed ceramic tipped blades is that for high quality tissue, such as facial towels, the conventional ceramic blade is not able to keep the very demanding tissue characteristics for any prolonged time. The inspection of worn ceramic blades shows that the impact of the tissue is much closer to the leading edge in such case compared to the situation for less quality demanding tissue. This can be understood by considering the fact that the high softness is obtained by having a very high adhesion onto the dryer surface, the web detachment and its impact thereby being close to the leading edge of the creping blade. The consequence is that once again chipping, in this case minute microchips, develop at the tip of the ceramic blade, leading to a “rounded” edge. Consequently, the tissue properties decreases rapidly even when using a ceramic tipped blade. For such situation, the solution in line with U.S. Pat. No. 3,688,336 referenced above will be useless because the impact will occur in the unprotected “breaking-in space”.        
The use of creping blades tipped with thermally sprayed metal carbides, such as for example WC—Co or WC—Co—Cr is known. Such materials are less brittle than sprayed ceramic and therefore less sensitive to edge chipping. Nevertheless, the use of such materials should be avoided due to other drawbacks, namely:                There is a potentially higher wear rate of the dryer surface, and potentially high damage due to chatter marks if vibrations develop within the tissue machine and are transferred to the creping blade.        Metal carbides are constituted by a metal matrix with embedded carbides. Such situation may promote the micro-welding events between the blade and the dryer surface at the high temperatures present in the sliding contact. This may lead to transfer of material from the dryer to the blade, causing premature wear or damage to the surface of the high cost dryer cylinder or costly metallization.        Another limitation of metal carbides derives from their high thermal conductivity. The friction wear is creating a large amount of heat, adding to the temperature of the already hot dryer surface. Steel creping blades or creping blades made of a steel substrate with an edge coating of metal or metal carbide may obtain a blue color at about 10 mm from the tip, which count for temperature exceeding 300° C. On long blades (wide machines), the steel expands sufficiently to create waving of the blade, instabilities in the blade holder, difficulties to unload blades and possibly damages to the Yankee drum, in particular when unloading such a hot blade. This is the so-called hot waving behavior.        
Consequently, there is a need in the tissue industry for a creping blade with improved behavior, including the advantageous features of ceramic materials from a friction standpoint, but lacking the chipping drawbacks resulting from material brittleness.